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Estimation for Standard Deviation to Calculate Cpk

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  • #32345

    Dennis Yong
    Participant

    Hi all. Thanks very much for introduction Wheeler’s very interesting text which introduces the right and wrong ways for calculating standard deviation.
    Just one question which I hope you could enlighten me in the next question: in calculating standard deviation to calculate Cpk, let us say for x-bar charts, can I use the quadratic equation formulae, for ex, in EXCEL STDEV(All individual values), rather than using the estimate R-bar/d2?
    This applies to the Individual MR chart also: where I use STDEV(All individual values), rather than MR/d2?
    CheersDennis

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    #86329

    Wagh
    Participant

    Hi D,
    1. R bar / d2 is a population std dev calculated from rational subgroups whereas the excel one is a sample std dev.got for data which is random.
     So in my opinion they are diff. and can’t be used in place of one another.
    pls chk it out & post for the same data
    2. Individual MR chart – don’t know ?
    Regds,
    prasad

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